There was a discussion about how effective really are the HVAC filters, particularly when in an enclosed space, aka planes.
As far as I am aware, no air-handler filters (at least, none sold for general commercial or residential use in the United States) are rated to trap particles smaller than 0.3 microns. The COVID-19 virus is approximately 0.125 micron. Of course, that's the virus itself. My understanding is that as an infectious agent, the virus is almost always encapsulated in a droplet such as those emitted by a cough or sneeze. I suspect those droplets typically
are large enough to be captured by most of the filters that would be used in a commercial building, a home with forced-air ventilation, or an aircraft. I read somewhere that the filters used in commercial airplanes have a rating similar to those used in hospital operating rooms.
As an aside, I've also read that about half the air circulated in airline cabins is fresh, the other half recirculated. It's impractical to use 100 percent fresh air at the altitude most jets fly and provide an acceptable level of humidity for the cabin occupants.